Method and arrangements relating to satellite-based positioning

ABSTRACT

The present invention relates to methods and apparatuses as well as a measurement report signal for reporting measurements on ranging signals (RS 1 -RS 4 ) received by a mobile station from satellites (SV 1 -SV 4 ) or calculating a position based on such measurements, wherein each of said ranging signal comprises a stream ( 201 ) of data bits ( 202 ) spread by a spreading code ( 203 ). After synchronizing ( 501 ) to data bit edges in the stream of data bits on a ranging signal, a position in time modulo the data bit length for said stream of data bits with respect to a selected point in time is measured ( 502 ). The measured position in time could be used by the apparatus performing the measurements on the received ranging signals for calculating ( 504 ) the position of the mobile station. Alternatively the apparatus could transmit ( 503 ) a wireless signal including data representing said measured position in time, allowing the mobile station position to be determined in another apparatus ( 101 ).

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to positioning of mobileequipment by use of satellites and in particular to such positioningassisted by land based communication nodes.

DESCRIPTION OF RELATED ART

In recent years, determination of the geographic position of an object,equipment or a person carrying the equipment has become more and moreinteresting in many fields of application. One approach to solve thepositioning is to use signals emitted from satellites to determine aposition. Well-known examples of such systems are the Global PositioningSystem (GPS) (see e.g. [1]) and the coming GALILEO system. The positionis given with respect to a specified coordinate system as atriangulation/trilateration based on a plurality of received satellitesignals.

A stand-alone GPS receiver can obtain full locking to GPS satellitesignals, without having any other information about the system exceptnominal carrier frequency and the rules by which data carried by thesignals are modulated. Basically, the three-dimensional position as wellas a receiver clock bias to the satellite time have to be determined inthe position calculation step.

Assisted GPS (AGPS) has been defined as an enhancement of GPS (see e.g.3rd Generation Partnership Project (3GPP) specifications TS 25.331 or TS44.031 or Open Mobile Alliance (OMA) specifications for Secure UserPlane Location (SUPL)) for integration of GPS receivers into userequipment, i.e. mobile stations, of cellular communication systems.Assisted GPS in general aims at improving the performance of GPSreceivers in many different respects, including detection sensitivity,time to obtain a location estimate, accuracy and saving battery power.This is done by moving some functionality from the GPS receiver in themobile station to the network and hence only performing a subset of theGPS tasks in the GPS receiver itself.

There are two types of AGPS, Mobile Station (or User Equipment) basedand Mobile Station (or User Equipment) assisted. In Mobile Station basedAGPS, the location of a mobile station is calculated in the mobilestation using ranging signal measurement results determined by themobile station together with assistance data provided by the network. InMobile Station assisted AGPS (sometimes also referred to as Networkbased AGPS), the mobile station only measures and reports timing ofreceived ranging signals reflecting the pseudoranges to the SpaceVehicles (i.e. satellites). For both types of AGPS, the measured timingof the ranging signals are truncated modulo 1 ms which corresponds to adistance of 300 km. When calculating the mobile station location, eitherin the mobile station itself or in a network location server, thecomplete pseudoranges need to be reconstructed using apriori informationabout the mobile station location together with the ranging signalmeasurement results determined by the mobile station, in order tocompute the precise mobile station location.

The inventors of the present invention have identified a problem withAGPS in that measuring and reporting truncated timing of receivedranging signals may cause ambiguities when determining pseudoranges tospace vehicles, if the precision of the apriori information about themobile station position is too low, i.e. the uncertainty of the mobilestation position in said apriori information is too large. As a result,if an incorrect pseudorange is selected and used as a basis fordetermining the location of the mobile station, there will be asignificant error in the calculated mobile station position in the orderof e.g. 100 km.

U.S. Patent Application No. 60/545,175 by one of the inventors of thepresent invention describes one way of addressing this problem involvingdiscarding unlikely pseudo range values.

SUMMARY OF THE INVENTION

The problem dealt with by the present invention is providing increasedrobustness, in the context of satellite based positioning withassistance data, against ambiguous pseudorange reconstruction.

An advantage afforded by the invention is increased robustness againstambiguous pseudorange reconstruction in connection with satellite basedpositioning with assistance data such as Assisted GPS (AGPS).

Another advantage of the invention is that the increased robustness isachieved without reducing the detection sensitivity.

Yet another advantage of the invention is that the increased robustnessis achieved with an insignificant increase in processing delays.

The invention will now be described in more detail with reference toexemplary embodiments thereof and also with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an example scenario of Mobile Stationassisted AGPS in which the present invention is applied

FIG. 2 is a diagram illustrating C/A code and navigation data bits inGPS ranging signals

FIG. 3 is a block diagram illustrating the format of GPS navigation datain GPS ranging signals

FIG. 4 is a diagram illustrating time in different parts of the systemillustrated in FIG. 1

FIG. 5 is a flow diagram illustrating basic methods according to theinvention for measuring ranging signals from satellites and calculatinga position based on measurements on ranging signals from satellitesrespectively.

FIG. 6 is a schematic block diagram of a mobile station according to afirst exemplary embodiment of the invention.

FIG. 7 is a flow diagram illustrating processing performed by the mobilestation of FIG. 6.

FIG. 8 is a block diagram illustrating correlation in connection withbit edge synchronization.

FIG. 9 is a schematic block diagram of a location server.

FIG. 10 is a block diagram illustrating an exemplary embodiment of ameasurement report signal format

FIG. 11 illustrates a worst case scenario for deriving a boundary forthe initial location uncertainty which prior art AGPS can handle inorder to still provide unambiguous pseudorange reconstruction.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 illustrates a non-limiting example scenario in which the presentinvention may be applied. In this example scenario a basic wirelesscommunication system SYS1 together with the Global Positioning System(GPS) is used to provide Mobile Station assisted AGPS. The exemplarywireless communication system SYS1 illustrated in FIG. 1 is a UniversalMobile Telecommunication System (UMTS). The communication system SYS1includes a network part NET1 and User Equipment (UE), alternativelyreferred to as mobile stations (MS). The network part NET1 comprises acore network CN1 and a UMTS Terrestrial Radio Access Network (UTRAN)RAN1. The core network CN1 includes a Mobile services Switching Center(MSC) node MSC1 that provides circuit-switched services and a GeneralPacket Radio Service (GPRS) node SGSN1, sometimes referred to as aServing GPRS Support node (SGSN), which is tailored to providepacket-switched type services.

Each of the core network nodes MSC1 and SGSN1 connects to the radioaccess network RAN1 over a radio access network interface referred to asthe Iu interface. The radio access network RAN1 includes one or moreradio network controllers (RNCs). For sake of simplicity, the radioaccess network RAN1 of FIG. 1 is shown with only one radio networkcontroller node RNC1. Each radio network controller is connected to andcontrols a plurality of radio base stations (RBSs). For example, andagain for sake of simplicity, FIG. 1 only illustrates a first radio basestation node RBS1 and a second radio base station node RBS2 connected tothe radio network controller node RNC1. The interface between the radionetwork controller RNC1 and the base stations RBS1 and RBS2 is referredto as the Iub interface. Mobile stations, such as mobile station MS1shown in FIG. 1, communicate with one or more radio base stationsRBS1-RBS2 over a radio or air interface referred to as the Uu interface.

Each of the radio interface Uu, the Iu interface and the Iub interfaceare shown by dashed lines in FIG. 1.

In FIG. 1, the GPS system is represented by Space Vehicles, i.e.satellites, SV1-SV4. Each Space Vehicle SV1-SV4 transmits acorresponding ranging signal RS1-RS4. Please note that for sake ofsimplicity, only four Space Vehicles SV1-SV4 are illustrated in FIG. 1.

When determining the position of the mobile station MS1 in FIG. 1 usingmobile station assisted AGPS, the mobile station MS1 receives assistancedata from and reports measurement results to a location server 101.Based on the reported measurement results and apriori information onwhere the mobile station is located, the location server calculates thelocation of the mobile station MS1. Depending on how a location serveris connected to a cellular network, AGPS can be divided into twocategories, “control plane solutions to AGPS” and “user plane solutionsto AGPS”.

In “control plane solutions to AGPS”, the location server functionality(which may be implemented in a separate location server node, sometimesreferred to as a Serving Mobile Location Center (SMLC) or StandaloneSMLC (SAS), or integrated together with other functionality in othernetwork nodes such as radio network controllers) is tightly integratedwith the cellular network and assistance data and measurement resultsare communicated using so called control plane signaling. This solutionis further characterized in that typically the location server wouldreceive information of in which cell a mobile station is currentlyoperating and the location server would apply this information as theapriori location of the mobile station when calculating the location ofthe mobile station. Hence the uncertainty in the apriori locationinformation corresponds to the cell size.

In “user plane solutions to AGPS”, the location server functionality isless closely integrated with the cellular network and assistance dataand measurement results are communicated using so called user planesignaling, i.e. ordinary user data packets are used to convey thisinformation transparently to the cellular network. This solution isfurther characterized in that the location server would not receiveinformation of in which cell a mobile station is located or at leastwould not always be able to associate a given cell identity with aspecific geographical area corresponding to the area covered by thecell. Hence, for user plane solutions to AGPS, the uncertainty in themobile station apriori location information may be significantly largerthan the cell size and may correspond to e.g. the size of the country inwhich the mobile station is currently operating.

In the example scenario of FIG. 1, a user plane solution to AGPS isillustrated where the location server 101 is connected to the cellularnetwork NET1 via an Internet Protocol (IP) based packet data network102.

The GPS Space Vehicles SV1-SV4 transmit ranging signals RS1-RS4 with aspectrum centered at 1575.42 MHz. FIG. 2 illustrates how each rangingsignal RS1-RS4 includes a stream 201 of navigation data bits 202 thatare spread by a spreading code defined by a so-called Coarse/Acquisition(C/A) code 203 that is unique for the Space Vehicle transmitting thesignal. The C/A code 203 has a length of 1023 chips and a chip durationof 1/1.023×10⁶ s, i.e. the C/A code comprises a sequence of +/−1 thatchanges at a rate of 1.023×10⁶ Hz and repeats itself every 1 ms. Thenavigation bits 202 have a bit period of 20 ms, i.e. corresponding to 20C/A code repetitions.

The navigation data includes among other things a set of so-calledephemeris parameters that enables the receiver to calculate the preciseposition of the satellites at the time of signal transmission. Theprecise time of transmission can also be read from the navigation data.

FIG. 3 illustrates more in detail how the navigation data is furtherdivided into 5 subframes 301-305, each of length 6 seconds. Eachsubframe 301-305 is divided into 10 words each of length 0.6 seconds andcontaining 30 data bits. A time stamp, GPS Time Of Week (TOW), istransmitted in the second word, the Handover Word (HOW), of everysubframe 301-305. The indicated time is the time of transmission at theend of the subframe in question. The TOW is thus repeated every 6seconds.

Each ranging signal RS1-RS4 basically defines a clock which is measuredby the mobile station MS1. The clock indicates the time of signaltransmission. If the mobile station MS1 knows the GPS system time, thenthe clock reading can directly be used to determine the time delay, andhence the range from the Space Vehicle transmitting the ranging signalto the mobile station MS1. By measuring three ranges and utilizing theknowledge about Space Vehicle locations at time of transmission, thelocation of the mobile station MS1 in three dimension can then bedetermined. However normally the mobile station MS1 does not haveknowledge about precise GPS system time, so one more measurement isneeded to eliminate the mobile station clock bias.

The sequences of FIG. 4 illustrate clock relations (expressed inmilliseconds) for different parts of the system illustrated in FIG. 1.Each Space Vehicle SV1-SV4 carry precise atomic clocks to maintain clockstability. The Space Vehicle transmissions are however not perfectlysynchronized to GPS system time as illustrated in FIG. 4. In FIG. 4,sequence 401 represents the GPS system time, sequence 411 represents theclock of Space Vehicle 1, sequence 41N represents the clock of SpaceVehicle N, sequence 402 represents the clock of mobile station MS1 ofFIG. 1 while sequences 421 and 42N respectively represent time as readin the ranging signals from Space Vehicle 1 and Space Vehicle Nrespectively received by the mobile station MS1. By drawing a verticalline 431 through the timing diagram one may obtain a snapshot of allclock readings as observed in various points in space. GPS system time401 is defined as an ensemble average based on a set of ground stationclocks and a subset of Space Vehicle clocks. As demonstrated in FIG. 4,the individual Space Vehicle clocks 411 and 41N and the mobile stationclock 402 are slightly offset (see SV clock biases 412 and 413 andmobile station clock bias 414 respectively) compared to GPS system time401. A model for the individual offsets of the Space Vehicle clocks istransmitted as part of the navigation message from each Space vehicle.When the signals reach a point on the earth surface (e.g. the currentlocation of mobile station MS1), they have been delayed with an amountdepending on the range from the Space Vehicle in question to said pointon the earth surface. The delay is typically 60-85 milliseconds (ms) asillustrated by the clock readings in FIG. 4.

When determining the position of a mobile station using AGPS, the mobilestation measures the C/A code boundary locations position in time withrespect to a selected point in time, i.e. the C/A code phase, for thereceived ranging signals. The C/A code phase is determined modulo 1 ms(i.e. one C/A code period).

A mobile station implementing mobile station based AGPS calculates itsposition at the selected point in time based on the measured C/A codephases of the received ranging signals and assistance data (receivedfrom the network) including space vehicle ephemeris and clock correctiondata together with apriori information about the mobile stationlocation.

A mobile station, such as mobile station MS1 in FIG. 1, implementingmobile station assisted AGPS instead transmits a wireless signalreporting the C/A code phases (expressed in terms of whole andfractional chips of the C/A code from the selected point in time untilthe beginning of the next C/A code repetition) for the received rangingsignals together with an estimate of the GPS system time correspondingto the selected point in time. Based on the information reported by themobile station and apriori information about the mobile stationlocation, a location server, such as location server 101 in FIG. 1,either in the cellular network or in another network calculates theposition of the mobile station.

The inventors of the present invention have recognized that the AGPS wayof measuring the C/A code phases modulo 1 ms and hence characterizingeach ranging signal by time mod 1 ms, causes problems when theuncertainty in the apriori information of the mobile station location istoo large. As demonstrated in APPENDIX 1, when the apriori locationuncertainty is more than 75 km, the so called pseudorange to a spacevehicle can not be unambiguously reconstructed.

The problem of ambiguous pseudorange reconstruction due to truncatedmeasurement of ranging signal timing in combination with too largeuncertainties in apriori location could be addressed by having a mobilestation measure ranging signal timing for each ranging signal withoutany truncation. This would however require decoding Time Of Weekinformation on each measured ranging signal which significantlyincreases the processing delays and may also decrease the detectionsensitivity of the GPS receiver integrated in the mobile station sinceit is significantly more difficult to decode the Time Of Weekinformation than to detect the C/A code boundaries.

The present invention addresses the above elaborated problem byproviding ways of significantly reducing the risk that, in the contextof AGPS (both mobile station based and mobile station assisted AGPS),the apriori location uncertainty of a mobile station causes ambiguouspseudorange reconstruction. At the same time the invention also avoidsthe need for decoding Time Of Week on each measured ranging signal andthe associated disadvantages.

FIG. 5 illustrates basic methods according to the invention forreporting measurements on ranging signals received by a mobile stationfrom satellites (i.e space vehicles) and for calculating a mobilestation position based on such measurements respectively, wherein eachof said ranging signals comprises a stream of data bits (e.g. the GPSnavigation data bits) spread by a spreading code (e.g. the GPS CoarseAcquisition code). Both methods include performing steps 501 and 502 forat least one of the received ranging signals, while the last step of thebasic methods differs.

At step 501, synchronizing to data bit edges in the stream of data bitsis performed. This would typically involve determining/identifying databit edge positions modulo the data bit length (i.e. modulo 20 ms forAGPS).

The position in time modulo the data bit length for the stream of databits with respect to a selected position in time is measured at step502.

After steps 501 and 502, the basic method for reporting measurementsincludes a further step 503 of wirelessly transmitting a signalincluding data representing said position in time measured at step 502.

After steps 501 and 502, the basic method for calculating the mobilestation position includes a further step 504 of calculating said mobilestation position utilizing said measured position in time for saidstream of data bits with respect to said selected point in time andassistance data received by the mobile station via at least one basestation of a wireless communication network. The received assistancedata would include ephemeris parameters and clock corrections of thesatellites transmitting the received ranging signals which makes itpossible to calculate the position of the satellites at time of rangingsignal transmission. The received assistance data would further includean apriori estimate of the mobile station position.

Typically said synchronizing and measuring steps 501 and 502 areperformed for plural received ranging signals, preferably all detectedranging signals. The signal transmitted at step 503 would then includedata representing the measured positions in time with respect to saidselected point in time for each of said plural received ranging signalswhile the position calculation performed at step 504 would utilize themeasured positions in time with respect to said selected point in timefor each of said plural received ranging signals.

Embodiments of the basic method for reporting measurements includingsteps 501, 502 and 503 could e.g. be used to implement processing in amobile station for supporting mobile station assisted AGPS where, basedon the measurement results reported by the mobile station, the actualposition calculations are performed in a node somewhere on the networkside.

Embodiments of the basic method for calculating a mobile stationposition including steps 501, 502 and 504 could e.g. be used toimplement processing in a mobile station for supporting mobile stationbased AGPS.

Applying the methods of FIG. 5 in the context of AGPS (either mobilestation assisted AGPS or mobile station based AGPS) would cause a mobilestation to measure timing of received ranging signals modulo 20 ms (ascompared to modulo 1 ms for prior art AGPS). A 20 ms truncation enablesunambiguous reconstruction of complete pseudoranges if the aprioriuncertainty of the mobile station location is less than 1500 km (ascompared to less than 75 km when applying the prior art 1 mstruncation).

First exemplary embodiments of a method and an apparatus for reportingranging signal measurements implemented in the mobile station MS1 ofFIG. 1 together with a first exemplary embodiment of an apparatus forposition calculation according to the invention are illustrated in FIGS.6-9. A first exemplary embodiment of a measurement report signalaccording to the invention is illustrated in FIG. 10.

FIG. 6 is a block diagram illustrating the structure of the mobilestation MS1 according to this exemplary embodiment of the invention. Themobile station MS1 includes a cellular communication module 601, apositioning module 602, a GPS RF front end 603, an antenna 604 forcommunication with the cellular network and a GPS antenna 605. Thepositioning module 602 includes a CPU 612, memory 610 and a DigitalSignal Processor (DSP) 611. The cellular communication module 601wirelessly receives assistance data from the cellular network andwirelessly transmits measurement results to the cellular network viabase stations in the cellular network. The assistance data could consistof ephemeris and clock corrections for visible satellites, anapproximate location of the mobile station MS1 and an approximate GPSsystem time. Alternatively the assistance data could contain explicitassistance data intended only for assisting the correlation processing.The communication module 601 forwards received assistance data to thepositioning module 602 using the interface 606 while measurement resultsare provided from the positioning module 602 to the communication module601 using interface 613. The communication module 601 also provides theGPS RF front end 603 and the positioning module 602 with a clockreference 607. The GPS RF front end module 603 is controlled by thepositioning module 602 using interface 608.

FIG. 7 illustrates processing performed by the mobile station MS1 whenit receives a positioning request.

When the positioning module 602 receives a positioning request from thecommunication module 601, it requests the GPS RF front end 603 toprovide GPS signal samples at step 701. The GPS RF front end 603receives the GPS frequency band through the antenna 605, downconvertsthe signal to baseband, separates the signal into in-phase (I) andquadrature (Q) components, samples and converts the signals into digitalformat, and outputs these to the positioning module 602 throughinterface 609. The positioning module 602 stores the received I and Qdata in memory 610.

Steps 702-703 define processing performed on each individual rangingsignal RS1-RS4 which is included in the measurement report transmittedat step 707. Please note that even though FIG. 7 illustrates sequentialprocessing (see step 704) of each individual ranging signal, processingrelated to different ranging signals are preferably performed inparallel.

A ranging signal y as a function of time t received from an arbitrarySpace Vehicle SV1-SV4 by the mobile station MS1 can in a simplified waybe written:y(t)=a·c(t−τ)·d(t−τ)·exp{i·(ω₀ t+ω _(d) t+φ)}+e(t)  (20)

Here a is the amplitude of the received signal, c(t) is the C/A code ofthe Space Vehicle and d(t) is the navigation data bit stream (see FIG.2). The term τ is the unknown delay of the signal which is a function ofthe distance from the Space Vehicle to the position of mobile stationMS1, ω₀ is the GPS carrier frequency, ω_(d) is the Doppler frequency ofthe signal, φ is an unknown phase and e(t) noise.

At step 702 the C/A code boundaries of a ranging signal are determinedby the Digital Signal Processor 611 in the positioning module 602 usingcorrelation that test all possible code phase and Doppler shifts for theranging signal.

Once the C/A code boundaries of a ranging signal have been determined atstep 702, processing of said ranging signal continues at step 703 by theDigital Signal Processor 611 in order to synchronize to the bit edges ofthe navigation data bit stream of this ranging signal. Bit edgesynchronization amounts to determine the data bit transitions of thed(t) sequence. There are several known ways of performing bit edgesynchronization in the literature (see e.g. chapter 8 of [2]). Oneexample of how bit edge synchronization could be performed would be tofirst despread the received data, leaving raw “pseudobits” with a bitrate of 1 ms (i.e. corresponding to each C/A code repetition). Note thatthere are 20 pseudobits per navigation data bit. In terms of formula(20) (and ignoring the noise component) this could be expressed ass(kT)=a·d(kT−τ)·exp(iφ); T=0.001 s, k=1, 2, . . . , N  (21)

Then, as illustrated by FIG. 8, the pseudobits are fed into a first setof accumulators 801 that sum up twenty consecutive pseudobits. Afterthis the sum is squared, and a new summation commences in a second setof accumulators 802. This procedure is repeated M times. M may beadapted to the current Signal-to-noise ratio or selected as a fixedvalue, e.g. M=50. All this is done for 20 different delays of thepseudobit sequence. The output bin, i.e. the accumulator in the secondset of accumulators 802, that maximizes the accumulation sum determinesthe data bit edge for the Space Vehicle in question. This can beexpressed as a number between 0 and 19 that will hereafter be calledinteger code phase (“icp”), since it counts the integer number of C/Acode periods since the latest navigation data bit boundary.

Once steps 702 and 703 have been initially completed for a rangingsignal, the DSP 611 maintains synchronization with said ranging signalby tracking changes in C/A code boundary/Bit edge timing of said rangingsignal.

If more ranging signals need to be acquired (an alternative YES at step704) steps 702 and 703 are repeated for a next ranging signal (asalready discussed, the processing of steps 702-703 are preferablyperformed in parallel for several ranging signals and not sequentiallyas indicated by FIG. 7). The decision in step 704 on whether moreranging signals should be acquired or not could be based on the numberof ranging signals acquired so far (at least 3 or preferrably 4 rangingsignals should be acquired, but acquiring more ranging signals wouldimprove the precision of the calculated position) and timingrequirements (the response time for providing a measurement reportsignal could be configured by a parameter to e.g. within 16 seconds ofreceiving a positioning request).

If enough ranging signals have been acquired (an alternative NO at step704), the GPS Time Of Week (TOW) of a selected point in time isestimated at step 705. Please note that preferably, as soon as steps702-703 have been completed for a first ranging signal, step 705 isperformed based on said ranging signal in parallel with acquiringadditional ranging signals.

There are several alternatives for how step 705 may be performed.Typically TOW estimation is based on determining the TOW transmitted inthe so called Handover Word (HOW) of one ranging signal (see FIG. 3),preferably the first acquired ranging signal, and then compensating forthe propagation delay from signal transmission by the Space Vehicleuntil signal reception by the mobile station MS1.

Determining the transmitted TOW can be performed by direct decoding ofthe transmitted TOW. This alternative implies that data is demodulatedat a rate of 20 ms and normally requires that subframe boundaries aredetermined followed by decoding of the Handover Word, from which theTOW, ie the transmission time t_(ti), can be derived. Each subframe hasa length of 6 s, so this procedure may require that approximately 8seconds of navigation data is collected. TOW demodulation works down toapproximately −172 dBW, assuming 0 dB antenna and is in fact thelimiting factor for detection sensitivity.

Alternatively, the transmitted TOW can be determined by reconstructionusing correlation techniques. This procedure also requires thatdemodulated data bits are generated, but instead of direct decoding,correlation is made with known transmitted navigation data bits (e.g.the contents of the so-called Telemetry Word and the HOW word which maybe sent to the mobile station as part of the assistance data). Thisrequires that the GPS time is a priori known to within a few seconds.This procedure works to somewhat lower signal levels than direct TOWdecoding, but most likely the performance is limited by the trackingloops that may loose lock at such low signal levels. Typically phaselocked loops or automatic frequency control loop are employed for this.But it is expected that this will work down to say −179 dBW.

Compensating for the propagation delay could be performed by applying anexpected average propagation delay of 77 ms. Alternatively a moreaccurate propagation delay compensation can be derived from assistancedata received from the cellular network by the mobile station MS1according to the principles elaborated in copending US patentapplication by inventors Ari Kangas and Janos Toth-Egetö filed Sep. 29,2004.

At step 706, the positions in time with respect to the selected point intime are measured for navigation data bit edges adjacent to the selectedpoint in time in each acquired ranging signal. More specifically, foreach acquired ranging signal, the position of the closest bit edgepreceding the selected point in time is measured at step 706 byregistering the number of whole and fractional chips from the selectedpoint in time until the next C/A code boundary and additionally theinteger number of C/A code periods between the selected point in timeand the closest preceding navigation data bit edge. By measuring the C/Acode phase shifts (whole and fractional chips) and the integer C/A codephase (number of C/A code repetitions), each ranging signal phase isthus determined modulo the navigation data bit length (i.e. 20 ms).

Finally, at step 707 a measurement report signal is wirelesslytransmitted by the mobile station MS1 to the cellular network NET1.

FIG. 10 illustrates schematically an exemplary format for themeasurement report signal 1001 used in this exemplary embodiment of theinvention. Please note that FIG. 10 provides a simplified view focusingon data that is relevant for the present invention and the measurementreport signal would include additional data not illustrated in FIG. 10(e.g. as specified for the MEASURE POSITION RESPONSE message accordingto 3GPP TS 44.031 or the MEASUREMENT REPORT message according to 3GPP TS25.331)

The measurement report signal 1001 includes the following data for eachmeasured ranging signal:

Satellite ID 1002 identifying the particular satellite for which themeasurement data is valid.

the number of whole chips 1003 and fractional chips 1004 from theselected point in time until the next C/A code boundary;

the integer number of C/A code periods 1005 between the selected pointin time and the closest preceding navigation data bit edge.

The measurement report signal also includes the estimated TOW 1006 atthe selected point in time.

The measurement report signal is in the first exemplary embodiment ofthe invention transmitted as ordinary user data in the user planeaddressed to the location server 101. Hence the measurement reportsignal is transparently routed through the cellular network NET1 via theIP based network 102 to the location server 101.

FIG. 9 schematically illustrates the structure of the location server101. The location server includes a communication module 901 and apositioning module 903. The communication module 901 receives themeasurement report and forwards measurement data to the positioningmodule 903. The positioning module 903 calculates the location of themobile station MS1 using the provided measurement data (including themeasured timing information for each reported ranging signal) andapriori information on the mobile station location. The aprioriinformation can e.g. be derived from a Public Land Mobile Network (PLMN)identity included in the signal from the mobile station MS1 andindicating in which network the mobile station MS1 is operating. ThePLMN identity could e.g. be included as part of the cell identity of thecell in which the mobile station is currently operating. Using theprovided PLMN identity, the positioning module 903 could derive theapriori location information e.g. by retrieving the coordinates of thecentre of the country in which the mobile station MS1 is operating,together with a radius corresponding to the maximum distance from saidcentre until the border of said country from a table. The hierarchicalnature of cell identities could also be exploited, in particular forlarge countries, to identify a particular region within a country inwhich the mobile station is operating. Maintaining a table ofcentre/radius information for different countries, or regions withinsaid countries, is significantly less burdensome than trying to maintaina global data base with information on the geographical coordinates ofeach cell.

In the exemplary first embodiment of the invention, the positioningmodule 602 of the mobile station MS1 functions both as synchronizingmeans for synchronizing to data bit edges in the stream of data bits onreceived ranging signals as well as measuring means for measuring thepositions of the stream of data bits on the received ranging signalswith respect to a selected point in time, while the cellularcommunication module 601 functions as transmitting means for wirelesslytransmitting signals including data representing said measured positionsin time. In the location server 101, the communication module 901functions as means for receiving measurement report signals includingsaid measured positions in time while the positioning module 903functions as means for calculating mobile station positions based on thereceived measurement results.

Apart from the exemplary first embodiment of the invention disclosedabove, there are several ways of providing rearrangements, modificationsand substitutions of the first embodiment resulting in additionalembodiments of the invention.

An exemplary embodiment for use in the context of mobile station basedAGPS could be derived from the illustrated first embodiment of theinvention by essentially replacing step 707 of FIG. 7 with a step ofcalculating the position of the mobile station MS1 in the positioningmodule 602 of the mobile station MS1. Thus the calculations performed bypositioning module 903 of the location server 101 in the first exemplaryembodiment would instead be performed by positioning module 602 of themobile station MS1. An apriori estimate of the mobile station positiontogether with satellite ephemeris data and clock corrections would beprovided by the network as assistance data for use in the positioncalculations.

The invention could of course be applied both in the context of controlplane and user plane solutions to AGPS. As regards control planesolutions to AGPS, the invention is probably most interesting to applyin the context of extended range cells (in GSM, extended range cellscould have a radius of up to 100 km) or when cell identity positioning(which typically is used as a basis for determining apriori locationinformation) is not implemented in a network. Applying the invention incontrol plane solutions to mobile assisted AGPS would implymodifications of signaling messages used to report measurement resultsfrom mobile stations in order to include data defining the measuredranging signal phases modulo the navigation data bit length (i.e. 20ms). This could preferably be achieved by adding the integer number ofC/A code periods from the closest preceding navigation data bit edge assuggested for the measurement report signal of the first exemplaryembodiment. Examples of signaling messages that need to be modified arethe MEASURE POSITION RESPONSE message specified in 3GPP TS 44.031 andthe MEASUREMENT REPORT message specified in 3GPP TS 25.331.

There are of course several alternatives for how measured positions oftime modulo the data bit length of data bit streams in ranging signalscould be represented in a measurement report signal apart from theformat suggested above and in FIG. 10. One alternative would be toinclude the integer number of C/A code periods to the closest bit edgefollowing the selected point in time. Another alternative would be totranslate the integer number of code repetitions together with thenumber of whole and fractional chips into time expressed in whole andfractional milliseconds.

By using real time clocks of sufficient precision, e.g. the cellularsystem clocks that typically drift only a few nanoseconds per second andhave a long term stability better than 1 ms for a significant time, TimeOf Week estimation would not require decoding/reconstructing TOWtransmitted in a ranging signal for each positioning request. Also, analternative to performing TOW estimation would be to measure anadditional ranging signal for estimating the unknown ranging signalreception time and, for mobile station assisted AGPS, include data forthe additional ranging signal in the measurement report signal insteadof a TOW estimate.

In situations where it would be desirable to handle even larger apriorilocation uncertainties than 1500 km in connection with AGPS, the presentinvention could be combined with the teachings of U.S. PatentApplication No. 60/545,175 by performing measurements modulo thenavigation data bit length on received ranging signals as specified inthis application and then eliminating unlikely pseudoranges as disclosedin said US patent application.

Even though the invention in its first exemplary embodiment has beenapplied in the context assisted GPS, the invention may of course beapplied in connection with other satellite based positioning systemswhere the transmitted ranging signals includes data bits spread by aspreading code.

APPENDIX 1

This appendix illustrates why apriori location uncertainties of morethan 75 km, imply that pseudoranges can not be unambiguouslyreconstructed due to the 1 ms truncation of prior art AGPS.

FIG. 11 illustrates a worst case scenario where a mobile stationmeasures the complete clock tsv1 of a first Space Vehicle SV1, i.e.performs complete TOW reconstruction of the ranging signal received fromSpace Vehicle SV1, and the fractional (submillisecond) part of a secondSpace Vehicle SV2 clock tsv2, i.e. only determines the C/A code phasefor the ranging signal received from Space Vehicle SV2. The mobilestation is known to be located within a circle, e.g. corresponding tothe cell boundary of a serving cell, having a radius Δ, i.e. the apriorilocation uncertainty of the mobile station is Δ. The distance from SpaceVehicle SV1 to the centre of the circle is d1 while the distance fromSpace Vehicle SV2 to the centre of the circle is d2. The measurement isdone at (the unknown) time t0. The question is now: Under whatconditions is it possible to reconstruct unambiguously the integermillisecond part of the clock tsv2?

Clocks tsv1 and tsv2 at the tentative mobile station locations A and Bare now calculated:

At A:tsv1=t0−(d1−Δ)/c  (1)tsv2=t0−(d2+Δ)/c  (2)where c is the speed at which radio signals propagates in vacuum.Subtracting (1) from (2) and rearranging, results intsv2=tsv1+(d1−d2−2Δ)/c  (3)At B:tsv1=t0−(d1+Δ)/c  (4)tsv2=t0−(d2−Δ)/c  (5)Subtracting (4) from (5) and rearranging, results intsv2=tsv1+(d1−d2+2Δ)/c  (6)Combining (3) and (6) it follows that tsv2 lies in the intervaltsv2ε(tsv1+(d1−d2−2Δ)/c,tsv1+(d1−d2+2Δ)/c)  (7)

The size of this interval is 4Δ/c. In order to reconstruct the integermillisecond part of tsv2 unambiguously, it is required that the intervalis less than 1 ms. Hence4Δ/c<0.001  (8)which leads to the requirementΔ<c*0.001/4˜75 km  (9)

REFERENCES

-   [1] Navstar GPS Space Segment/Navigation user Interfaces,    ICD-GPS-200, Revision IRN-200C-003, 11 Oct. 1999.-   [2] Parkinson, Spilker Global Positioning System: Theory and    Applications, Volume 1, AIAA, 1996.

1. A method in a mobile station for reporting measurements on rangingsignal received by the mobile station from satellites, said rangingsignals each comprising a stream of data bits spread by a spreadingcode, said method including the steps of for at least one of thereceived ranging signals: synchronizing the mobile station to data bitedges in the stream of data bits, wherein the synchronizing furthercomprises identifying data bit edge positions modulo a data bit length;measuring, by the mobile station, a position in time modulo the data bitlength for the stream of data bits with respect to a selected point intime, wherein said measuring comprises determining the position in timewith respect to said selected point in time for an adjacent bit edge inthe stream of data bits by registering a number of whole and fractionalchips from said selected point in time until a next course/acquisition(C/A) code boundary and an integer number of C/A code periods betweensaid selected point in time and a closest preceding navigation bit edge;and transmitting a signal including data representing said measuredposition in time.
 2. A method according to claim 1, wherein saidsynchronizing and measuring steps are performed for plural receivedranging signals and said wirelessly transmitted signal includes datarepresenting the measured positions in time with respect to saidselected point in time for each of said plural received ranging signals.3. A method according to claim 1, wherein said adjacent bit edge is theclosest following bit edge after said selected point in time.
 4. Amethod according to claim 1, wherein said adjacent bit edge is theclosest bit edge preceding said selected point in time.
 5. A methodaccording to claim 1, wherein the satellites are part of the GlobalPositioning System.
 6. A method according to claim 5, wherein saidspreading codes are Coarse Acquisition codes and the streams of databits are streams of Navigation data bits according to the format of theGlobal Positioning System specifications.
 7. A method according to claim6, wherein said step of synchronizing includes the substeps of:determining Coarse Acquisition code boundaries; determining navigationdata bit edges.
 8. A method in a mobile station for calculating themobile station position based on measurements on ranging signalsreceived by the mobile station from satellites, said ranging signalseach comprising a stream of data bits spread by a spreading code, saidmethod including the steps of for at least one of the received rangingsignals: synchronizing the mobile station to data bit edges in thestream of data bits, wherein the synchronizing further comprisesidentifying data bit edge positions modulo a data bit length; measuring,by the mobile station, a position in time modulo the data bit length forthe stream of data bits with respect to a selected point in time;calculating said mobile station position utilizing said measuredposition in time for said stream of data bits with respect to saidselected point in time and assistance data received by the mobilestation via at least one base station of a wireless communicationnetwork.
 9. A method according to claim 8, wherein said synchronizingand measuring steps are performed for plural received ranging signalsand said position calculation is performed utilizing the measuredpositions in time with respect to said selected point in time for eachof said plural received ranging signals.
 10. An apparatus for performingmeasurements on ranging signals received by a mobile station fromsatellites, said ranging signals each comprising a stream of data bitsspread by a spreading code, said apparatus including: synchronizingmeans for synchronizing a mobile station to data bit edges in the streamof data bits of at least one received ranging signal, wherein thesynchronizing comprises identifying data bit edge positions modulo adata bit length; measuring means for measuring a position in time modulothe data bit length for the stream of data bits of said at least onereceived ranging signal with respect to a selected point in time,wherein said measuring means are adapted to determine the position intime with respect to said selected point in time for an adjacent bitedge in the stream of data bits by registering a number of whole andfractional chips from said selected point in time until a nextcoarse/acquisition (C/A) code boundary and an integer number of C/A codeperiods between said selected point in time and a closest precedingnavigation data bit edge, and transmitting means for transmitting asignal including data representing said measured position in time. 11.An apparatus according to claim 10, wherein said synchronizing andmeasuring means are adapted to operate on plural received rangingsignals and said transmitting means are adapted to include data in saidsignal representing the measured positions in time with respect to saidselected point in time for each of said plural received ranging signals.12. An apparatus according to claim 10, wherein said adjacent bit edgeis the closest following bit edge after said selected point in time. 13.An apparatus according to claim 10, wherein said adjacent bit edge isthe closest bit edge preceding said selected point in time.
 14. Anapparatus according to, claim 10, wherein the satellites are part of theGlobal Positioning System.
 15. An apparatus according to claim 14,wherein said spreading codes are Coarse Acquisition codes and thestreams of data bits are streams of Navigation data bits according tothe format of the Global Positioning System specifications.
 16. A mobilestation including an apparatus according to claim
 10. 17. An apparatusfor calculating a position of a mobile station based on measurements onranging signals received by the mobile station from satellites, saidranging signals each comprising a stream of data bits spread by aspreading code, said apparatus including: synchronizing means forsynchronizing a mobile station to data bit edges in the stream of databits of at least one received ranging signal, wherein the synchronizingcomprises identifying data bit edge positions modulo a data bit length;measuring means for measuring a position in time modulo the data bitlength for the stream of data bits of said at least one received rangingsignal with respect to a selected point in time, calculating means forcalculating said position of the mobile station utilizing said measuredposition in time for said stream of data bits with respect to saidselected point in time and assistance data received by the mobilestation via at least one base station of a wireless communicationnetwork.
 18. An apparatus according to claim 17, wherein saidsynchronizing and measuring means are adapted to operate on pluralreceived ranging signals and said calculating means are adapted toutilize the measured positions in time with respect to said selectedpoint in time for each of said plural received ranging signals.